This invention relates to a toner for development of electrostatic images, more particularly to a microcapsule type toner which can preferably be used as the pressure fixing type.
In recent years, according to electrophotography, electrostatic printing or electrostatic recording, it has widely been practiced to form an image based on image information, develop the image with a toner as developer to give a toner image and then fixing the toner image, generally after transferring the toner image to a transfer paper, thereby forming a visible image.
In the prior art, as the toner for development of electrostatic images, use is made widely of powdery toners comprising thermoplastic resins as binders having incorporated colorants such as carbon black, etc. dispersed therein. If it is a two-component toner, it is stirred together with carriers such as iron powder, glass beads, etc., while the toner itself is stirred in the case when it is a one-component toner containing fine powder of magnetic material thereby to effect triboelectric charging, of which electrostatic force is utilized for development of electrostatic charges and the resultant toner image is, for example, transferred and thereafter fixed by heating with a heating roller, etc.
However, in such a toner, toner particles will be broken during stirring for triboelectric charging to form fine powdery toner. As the result, the quality of the visible images obtained will become inferior or exchange of the toner for new one may be required at earlier stage. Moreover, for accomplishing fixing according to heating fixing system, a long waiting time is necessary before the temperature of the fixer reaches a desired setting temperature and an enormous amount of energy is required for heating. Further, there also may be trouble such as the danger of fire when jamming of papers occurs. Besides, to ensure achievement of fixing, it is necessary to satisfy considerably severe conditions with respect to temperature conditions and others.
Under the situation as described above, investigations have recently been made about so-called microcapsules for use as the toner for development of electrostatic images. This microcapsule type toner is a powdery material comprising colored particles encapsulating a fluid core material or a soft solid within microparticulate resin capsules. When this toner is employed, fixing can be effected by applying pressure by means of a pressing roller thereby to have the capsules ruptured, so to speak, whereby no heating is required to alleaviate greatly the problems in heating fixing as described above.
As such pressure fixable microcapsule type toners, various techniques have been known as disclosed in Japanese Unexamined Patent Publication Nos. 91724/1976, 119937/1977, 118249/1979 and 64251/1980. However, problems such as off-set to pressurizing roller, durability, stability and storability remain to be solved. In particular, in pressure fixing onto a plain paper, the problems are great and there are also a number of tasks to be overcome.
More specifically, the fixing component within a microcapsule type toner may be too rigid or contrariwise susceptible to plastic deformation, giving no satisfactory fixing in either case. Also, in a capsule type toner of which microcapsule core material is prepared according to the crushing step of a soft solid, it is necessary to take a manufacturing procedure requiring cooling of the solid during crushing. Further, recourse is sometimes made to the method for encapsulation of a soft material by means of a two-nozzle atomizer which is actually very difficult to handle. Thus, there are involved a large number of problems in production other than fixing characteristic.
The present inventors have made extensive studies to solve such various problems and obtained the knowledge described below. The process of pressure fixing of a microcapsule toner by means of a pressurizing roller generally comprises first rupturing of the outer wall of the capsule by pressurization, then flowing out of the inner core material and pressure fixing of the core flowed out onto the member for fixing. During this process, fixing is effected with the core material participating primarily in pressure fixing being in the form including also the ruptured outer wall therein.
When a core material is fixed onto a member for fixing, the fixing mechanism may be considered to proceed first by way of primary bonding, namely bonding between atoms of both, or secondary bonding, namely intermolecular bonding between both, or further anchoring effect, etc., subsequently or simulataneously followed by coalescing process between core material and core material, core material and outer wall, and outer wall and outer wall, namely mutually between toners, which process may also be considered to proceed according to substantially the same fixing mechanism as described above.
Concerning pressure fixing, when the fixing component in the toner has a very low viscosity, in the case of using, for example, paper as the member for fixing, a visible image can be formed with satisfactory fixing by penetration from its surface fiber into the inner portion. At the same time, however, off-set to the pressurizing roller or penetrability into the outer wall are also high to ensure problems in storability and stability. On the other hand, when the fixing component in the toner has low penetrability, for examble, a high viscosity, it prevents not only fixing onto the member for fixing, but also drop-off of the toner from the member for fixing through coalescence between toners in its fixing mechanism.
Accordingly, for obtaining high pressure fixing characteristics, it is required to increase the effective contact area between toners and the member for fixing and also the effective contact area between toners, and also at the same time required to prevent drop-off of the toners from the member for fixing or drop-off of the toners once sufficiently contacted through the self-agglomerating force of the fixing component itself of the toner.
The present inventors have made extensive studies in view of the facts as mentioned above and have found a technique to improve the pressure fixing characteristic of a microcapsule toner by incorporating a substance having a glass transition point (hereinafter abbreviated as Tg) within the range of from -90.degree. C. to 5.degree. C. as the constituent of the core material based on the discovery that the visco-elastic characteristics of the pressure fixing component in the toner are controlling factors in the fixing characteristic of the toner. Further, as the result of further progress of the study, it has been found to be greatly effective in taking a greater effective contact area and providing high self-agglomerating force of the toner, respectively, to incorporate a substance having a Tg in the range of from -90.degree. C. to 5.degree. C. and a substance having a softening point (hereinafter abbreviated as Tsp) within the range of from 25.degree. C. to 180.degree. C. as the pressure fixing components in the constituents for core material, thus providing a toner which exhibits very high fixing characteristic.
In this context, as the technique referring to the glass transition point in the capsule toner of the prior art, there is known the technique disclosed in Japanese Unexamined Patent Publication No. 17739/1974. However, this technique gives only general description about the relation between the glass transition point and the fixing characteristic, and suggests no technical thought as disclosed in this invention that fixing characteristic is attempted to be improved by incorporating a substance or a mixed substance system having a glass transition point within specific range and a substance or a mixed substance having a softening point within specific range.